研究者詳細 - 和田　桂一

論文 - 和田　桂一

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Physically motivated X-ray obscurer models . 651 A58 - A69 2021年7月査読国際共著

Taisei Uzuo , Keiichi Wada , Takuma Izumi , Shunsuke Baba , Kosei Matsumoto , Yuki Kudoh . Circumnuclear Multi-phase Gas in Circinus Galaxy IV: Absorption Owing to High-J CO Rotational Transitions . The Astrophysical Journal915 89 2021年7月査読

Taisei Uzuo, Keiichi Wada, Takuma Izumi, Shunsuke Baba, Kosei Matsumoto, Yuki Kudoh . Circumnuclear Multi-phase Gas in Circinus Galaxy IV: Absorption Owing to High-J CO Rotational Transitions . Astrophysical Journal915 ( 2 ) 2021年7月査読

担当区分：責任著者掲載種別：研究論文（学術雑誌）

We studied the absorption features of CO lines against the continuum originating from the heated dust in the obscuring tori around active galactic nuclei (AGNs). We investigated the formation of absorption lines corresponding to the CO rotational transitions using three-dimensional non-LTE line transfer simulations considering the dust thermal emission. As in Papers I-III of this series, we performed post-processed radiative transfer calculations using the "radiation-driven fountain model"(Wada et al. 2016), which yields a geometrically thick obscuring structure around the nucleus. This model is consistent with the spectral energy distribution of the nearest type-2 Seyfert galaxy, the Circinus galaxy. We found that the continuum-subtracted channel maps of J = 4-3 and higher transitions show absorption regions along the disk midplane for an edge-on viewing angle. The spectra consist of multiple absorption and emission features, reflecting the internal inhomogeneous and turbulent structure of the torus. The deepest absorption feature is caused by the gas on the near side of the torus between r = 10 and 15 pc, which is located in front of the AGN-heated dust inside r ≃ 5 pc. We also found that a spatial resolution of 0.5-1.0 pc is necessary to resolve the absorption features. Moreover, the inclination angle must be close to the edge-on angle (i.e., ⪆85 ) to observe the absorption features. The findings of the present study imply that combining our radiation-hydrodynamic model with high-resolution observations of CO (7-6) by ALMA can provide new information about the internal structure of the molecular tori in nearby AGNs.

DOI： 10.3847/1538-4357/ac013d

Scopus

Johannes Buchner, Murray Brightman, Mislav Baloković, Keiichi Wada, Franz E. Bauer, Kirpal Nandra . Physically motivated X-ray obscurer models . Astronomy and Astrophysics651 2021年7月査読

掲載種別：研究論文（学術雑誌）

The nuclear obscurer of active galactic nuclei (AGN) is poorly understood in terms of its origin, geometry, and dynamics. Aims. We investigate whether physically motivated geometries emerging from hydro-radiative simulations can be dierentiated with X-ray reflection spectroscopy. Methods. For two new geometries, the radiative fountain model and a warped disk, we release spectral models produced with the ray tracing code XARS. We contrast these models with spectra of three nearby AGN taken by NuSTAR and Swift/BAT. Results. Along heavily obscured sightlines, the models present dierent 420 keV continuum spectra. These can be dierentiated by current observations. Spectral fits of the Circinus Galaxy favour the warped disk model over the radiative fountain, and clumpy or smooth torus models. Conclusions. The necessary reflector (NH 1025 cm2) suggests a hidden population of heavily Compton-thick AGN amongst local galaxies. X-ray reflection spectroscopy is a promising pathway to understand the nuclear obscurer in AGN.

DOI： 10.1051/0004-6361/201834963

Scopus

Keiichi Wada, Yusuke Tsukamoto, Eiichiro Kokubo . Formation of ``Blanets" from Dust Grains around the Supermassive Black Holes in Galaxies . The Astrophysical Journal909 ( 1 ) 96 - 105 2021年3月査読

Wada K. . Nature of inflows and outflows in AGNs . Proceedings of the International Astronomical Union359 297 - 306 2021年招待

Kudo Yuki, Wada Keiichi, Norman Colin . Multiphase Circumnuclear Gas in a Low-Beta Disk: Turbulence and Magnetic Field Reversals . The Astrophysical Journal904 9 - 23 2020年11月査読

Masatoshi Imanishi,Dieu D. Nguyen,Keiichi Wada,Yoshiaki Hagiwara, Satoru Iguchi,Takuma Izumi,Nozomu Kawakatu, Kouichiro Nakanishi, Kyoko Onishi . ALMA 0.''02-RESOLUTION OBSERVATIONS REVEAL HCN-ABUNDANCE-ENHANCED COUNTER-ROTATING AND OUTFLOWING DENSE MOLECULAR GAS AT THE NGC 1068 NUCLEUS . The Astrophysical Journal902 99 - 123 2020年10月査読

S. Aalto, N. Falstad, S. Muller, K. Wada 他 . ALMA resolves the remarkable molecular jet and rotating wind in the extremely radio-quiet galaxy NGC 1377 . Astronomy and Astrophysics640 104 - 119 2020年8月査読

Aalto S. . ALMA resolves the remarkable molecular jet and rotating wind in the extremely radio-quiet galaxy NGC 1377 . Astronomy and Astrophysics640 2020年8月

Kawakatu Nozomu, Wada Keiichi, Ichikawa Kohei . Obscuring Fraction of Active Galactic Nuclei Implied by Supernova and Radiative Feedbacks . The Astrophysical Journal889 ( 2 ) 84 - 93 2020年2月査読

Kawaguchi Takeru, Yutani Naomichi, Wada, Keiichi . Mass accretion toward black holes in final phase of galaxy mergers . The Astrophysical Journal890 ( 2 ) 125 - 136 2020年2月査読

Keiichi Wada, Yusuke Tsukamoto, Eiichiro Kokubo . Formation of Planets around Supermassive Black Holes in Active Galactic Nuclei . The Astrophysical Journal 2019年12月

Kudoh Yuki, Wada Keiichi . Magneto Rotational Instability in Magnetized AGN Tori . Galaxies6 ( 4 ) 139 2018年12月査読

記述言語：英語掲載種別：研究論文（学術雑誌）

It is widely believed that in active galactic nuclei (AGNs) a supermassive black hole with an accretion disk is surrounded by an optically and geometrically thick torus at sub-parsec scale. However, it is not clear how is the mass supply toward the central engine caused and how it is related with the internal structures of the tori. The magnetic field in the tori may contribute to the accretion process via the magneto-rotational instability (MRI). Using global three dimensional magnetohydrodynamic (MHD) simulations taking the effects of X-ray heating and radiative cooling into account studied the numerical resolution for azimuthal direction for MRI driving. We found that a strongly magnetized disk consisted of a cold ($< 10^3$ K) and warm ($10^4$ K) gas is developed in about 30 rotational periods. We also found in high resolution model that the mean azimuthal magnetic fields reverse their direction quasi-periodically. We confirmed that the typical wave length of the MRI should be resolved with a least 20 azimuthal grid cells.

DOI： 10.3390/galaxies6040139

Wada Keiichi, Yonekura Kensuke, Nagao Tohru . Circumnuclear Multiphase Gas in Circinus Galaxy. III. Structure of the Nuclear Ionized Gas . The Astrophysical Journal867 49 - 56 2018年11月査読

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our “radiation-driven fountain” model for the nearest type-2 Seyfert galaxy, the Circinus galaxy. We conducted “quasi-three-dimensional” spectral analysis using the spectral synthesis code CLOUDY and obtained the surface brightness distributions of lines, such as Hα, Hβ, [O III], [N II], and [S II] for the central 16 pc region. The ionized regions observed based on these lines show a conical morphology around the rotation axis, even if we do not phenomenologically postulate the presence of an optically thick “torus.” This region also shows non-uniform internal structures, reflecting the inhomogeneous structure of fountain flows. Using ionization diagnostic diagrams, we investigated the spectral properties of the ionized gas. The diagrams based on the line ratios of [N II]/Hα and [S II]/Hα show that most regions of the cone have the same properties as those in the narrow line regions (NLRs) in AGNs, whereas using [O I]/Hα, the central 10 pc regions are rather LINER-like. The gas density, temperature, and ionizing parameter in regions identified as “NLR” are typically n ∼ 300–1500 cm‑3, T ∼ (1–3) × 104 K, and U ∼ 0.01, respectively. The morphology and [O III] intensity are similar to the base of the observed [O III] cone in the Circinus galaxy, implying some physical connections with the origin of the ∼100 pc scale NLR.

DOI： 10.3847/1538-4357/aae204

Izumi, Takuma; Wada, Keiichi; Fukushige, Ryosuke; Hamamura, Sota; Kohno, Kotaro . Circumnuclear Multiphase Gas in the Circinus Galaxy. II. The Molecular and Atomic Obscuring Structures Revealed with ALMA . The Astrophysical Journal867 48 - 68 2018年11月査読

記述言語：英語掲載種別：研究論文（学術雑誌）

We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3–2) and [C I](1–0) lines, as well as their underlying continuum emission, from the central ∼200 pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of ∼6–15 pc. The lines and continuum-emitting regions consist of a circumnuclear disk (CND; 74 pc × 34 pc) and spiral arms. The distribution of the continuum emission revealed a temperature-dependent dust geometry and possibly polar dust elongation in the torus region. The molecular mass of the CND is {M}{{H}2}∼ 3× {10}6 {M}ȯ , with a beam-averaged H2 column density of ∼5 × 1023 cm‑2 toward the AGN position, which contributes significantly to the nuclear obscuration. The [C I](1–0)/CO(3–2) ratio at the AGN position is unusually high, suggesting an X-ray-dominated region-type chemistry. We decomposed the observed velocity fields into rotational and dispersion components, and revealed a multiphase dynamic nature in the r ≲ 10 pc torus region, i.e., the diffuse atomic gas is more spatially extended along the vertical direction of the disk than the dense molecular gas. Through comparisons with our model predictions based on the radiation-driven fountain scheme, we indicate that atomic outflows are the driver of the geometrical thickness of the atomic disk. This supports the validity of the radiation-driven fountain scheme in the vicinity of this AGN, which would explain the longstanding mystery of the physical origin of the AGN torus.

DOI： 10.3847/1538-4357/aae20b

Imanishi, Masatoshi; Nakanishi, Kouichiro; Izumi, Takuma; Wada, Keiichi . ALMA Reveals an Inhomogeneous Compact Rotating Dense Molecular Torus at the NGC 1068 Nucleus Authors . The Astrophysical Journal Letters 2018年2月査読

Wada Keiichi, Fukushige Ryosuke, Izumi Takuma, Tomisaka, Kohji . Circumnuclear Multi-phase Gas in the Circinus Galaxy. I. Non-LTE Calculations of CO Lines10.3847/1538-4357/aa9e53 . The Astrophysical Journal852 ( 2 ) 88 - 99 2018年1月査読

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：American Astronomical Society

In this study, we investigate the line emissions from cold molecular gas based on our previous “radiation-driven fountain model,” which reliably explains the spectral energy distribution of the nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central r≤slant 16 pc, in which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around J≃ 6, and its distribution suggests that the lines are not thermalized. However, for a given line of sight, the optical depth distribution is highly non-uniform between {τ }ν \ll 1 and {τ }ν \gg 1. The CO-to-H2 conversion factor ({X}{CO}), which can be directly obtained from the results and is not a constant, depends strongly on the integrated intensity and differs from the fiducial value for local objects. {X}{CO} exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a “torus.” In addition, we found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the interstellar medium.

DOI： 10.3847/1538-4357/aa9e53

Schartmann, M.; Mould, J.; Wada, K.; Burkert, A.; Durré, M.; Behrendt, M.; Davies, R. I.; Burtscher, L. . The life cycle of starbursting circumnuclear gas discs . Monthly Notices of the Royal Astronomical Society473 ( 1 ) 953 - 968 2018年1月招待査読

Aalto S. . Luminous, pc-scale CO 6-5 emission in the obscured nucleus of NGC 1377 . Astronomy and Astrophysics608 2017年12月

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